As uses for polymeric compositions have become more sophisticated, the demand has grown for polymers and polymer mixtures having physical properties tailored to the desired end use. Vast numbers of polymeric systems having 2, 3 or more monomers have been developed in order to achieve certain balances of properties. The degree that such properties can be controlled becomes increasingly complex as the number of different monomers in such systems increases. It has become more practicable, therefore, to modify and tailor polymer properties by combining preformed polymers of known composition and behavior in mixtures or blends. As the general rule, however, polymers which differ from each other significantly in composition are incompatible and do not mix well in a manner which enables them to share their properties or enhance the properties of one of the base polymers. Basically, this incompatibility results because the polymers do not combine on a molecular level which would be the case if the polymers were completely miscible with each other.
The miscibility between polymers has been studied extensively, as by Olabisi, et al, "Polymer-Polymer Miscibility", Academic Press, New York, (1979). The authors describe blends of miscible polymers which exhibit behavior similar to that which would be expected from a single phase system. This offers assurance of mechanical compatibility with enhanced tensile properties, noting that miscible polymer blends are usually useful over the entire composition range of components thereby offering great versatility in matching price/performance requirements.
The problem of blending normally incompatible polymers was addressed several years earlier by Murdock, et al., U.S. Pat. No. 3,236,914 (1966). The solution offered was to combine one linear polymer containing nitrogen atoms with another linear polymer having carboxylic acid groups, for example vinyl pyridine and acrylic acid or methacrylic acid. The so-called "neutral" comonomers could be selected from a long list of polymers including styrene and vinyl chloride or butadiene and methyl methacrylate.
Over two decades later, the problem of polymer miscibility was addressed in a similar fashion by Hsieh and Wong, J. Chem. I. Ch. E., 19(1), 17(1988) who studied phase separation between poly(vinyl acetate) and polystyrene. Miscibility was noted for copolymers based upon vinyl acetate/acrylic acid and styrene/4-vinyl pyridine. Also studied were blends of copolymers of styrene with acrylic acid and styrene with vinyl pyridine. The interaction between the polymer chains was said to be enhanced by introducing charge groups, namely the vinyl pyridine and acrylic acid into the system. The single phase was noted at 10% content of the charge groups with improved compatibility and a Tg of the blend centered between the Tg of the component copolymers at 30 mole percent of the charge group monomer. The reference teaches that each of the polymers of the blend must have one of the charge group monomers in its composition.
U.S. Pat. No. 4,332,917, Heslinga, et al., (1982) describes polymer alloys formed from one or more polymers having anhydride groups and one or more polymers able to hydrogen bond to the first polymer which must be protolyzed, for example, a copolymer of styrene and maleic anhydride and poly(vinyl acetate). These polymers are said to be miscible in all proportions at certain temperatures but at specific temperatures phase separation will take place. Physical interaction between the polymers is said to be improved by protolysis of the maleic anhydride portion of the copolymer in order to increase hydrogen bonding. This must be done after the polymer is in solution in an organic solvent. Relative proportions of the styrene and maleic anhydride are not disclosed but one example indicates that about equal amounts were charged to a polymerization.
Shiomi, et al., Macromolecules, 19, 2274 (1986) states that compatibility between high molecular weight polymers is unusual in blend systems, but that specific interaction between two polymers does improve the potential for miscibility. The authors studied random copolymer blends in which the blended copolymer had a common monomer, for example, vinyl acetate/ethylene and vinyl chloride/vinyl acetate copolymers. Another system studied was the copolymer of styrene and maleic anhydride blended with a copolymer of styrene and acrylonitrile.
Many polymer blends are disclosed in the literature without actually addressing the problem of compatibility. For example Japanese Patent Application 60042476 (1985) describes vibration proof materials which include fiber and resin made from vinyl acetate/ethylene copolymer and a styrene copolymerized with acrylic acid ester with a third monomer of acrylic acid being optional. The compositions disclosed are, however, outside the range of miscibility with vinyl acetate/ethylene copolymer (VAE). In like manner Japanese Patent Application 62070461 (1987) describes a varnish containing VAE having 10 to 40 weight percent ethylene and a copolymer of styrene with maleic acid, the VAE being in emulsion and the styrene copolymer in solution. In effect, this system would produce a mixture of emulsions so that the resultant cast film would consist of particles of VAE surrounded by the styrene resin and therefore would be phase separated. Also, U.S. Pat. No. 4,777,197, Nolken, et al. (1988) describes as a plastic dispersion a mixture of VAE containing 14 to 22% ethylene and a copolymer of styrene with maleic anhydride as a water soluble salt plus a dispersing agent. Since the dispersion would contain particles of the poly(vinyl ester), mixing would not occur on the molecular level.
Cruz-Ramos, et al., Macromolecules, 22, 1289 (1989) discloses that poly(vinyl chloride) is not miscible with either poly(vinyl acetate) or polyethylene, but does form miscible mixtures with some of the copolymers of vinyl acetate and ethylene containing about 15-55 weight percent ethylene. The authors discuss "miscibility windows" in homopolymer-copolymer blends, but do not go beyond the studies made with poly(vinyl chloride).